CS 4594 Broadband

CS 4594 Broadband

ATM GCRA and Leaky Bucket

From the ATM Forum

• The GCRA is used to define conformance to the traffic contract. For each cell that arrives at the specified point, the GCRA determines whether the cell conforms to the traffic contract of its connection.

Generic Cell Rate Algorithm

• GCRA = Generic Cell Rate Algorithm

• Defined by two equivalent methods– Virtual Scheduling– Leaky Bucket

GCRA – Virtual Scheduling

Arrival of cell k at t =ta(k)

TAT<ta(k)

TAT>ta(k)+L

TAT=TAT+I

TAT=ta(k)

Discard nonconforming cell

Yes

No

Yes

No

Accept conforming cell

k=k+1

Initially:

TAT=ta(1)

GCRA - Virtual Scheduling

Arrival of cell k at t =ta(k)

TAT<ta(k)

TAT>ta(k)+L

TAT=TAT+I

TAT=ta(k)

Discard nonconforming cell

Yes

No

Yes

No

Accept conforming cell

k=k+1

TAT-L TAT

fasttoo fast slow(slow)

(fast)

(too fast)

(not too fast)

CGRA(I,L)

TAT = Theoretical Arrival Time

TATTAT-LTATTAT-L

time

II

slowfasttoo fast

I I

)()()(),(

)()()()(,)(

)()()(,)(

)1(

fasttooLkTATktifkTAT

fastkTATktLkTATifIkTAT

slowktkTATifIkt

kTAT

a

a

aa

Burst Size

T

N 1

• GCRA(T, τ)• N = maximum number of contiguous conforming cells• T = targeted average time in GCRA• τ = limit tolerance in GCRA• δ = actual average arrival time

Leaky Bucket

• Smoothes traffic

• Regulates traffic

• Overflow = nonconforming

• Nonconforming can be tagged or discarded

GCRA – Leaky Bucket (Equiv to Virtual Scheduling)

Arrival of cell k at t =ta(k)

X’<0

X’>L

X=X’+ILCT=ta(k)

X’=0

Discard nonconforming cell

Yes

NoYes

No

Accept conforming cell

k=k+1

(slow)

(fast)

(too fast)

(not too fast)

X’=X-(ta(k)-LCT)

Initially:

X=0

LCT = ta(1)

Meanings:

X = Value of counter

X’ auxiliary variable

LCT = Last Conformance Time

L = bucket size

Explanation

• X is value of the counter.• When a cell arrives at time ta(k), X’ is set to the value of

what the counter would be: X’ = X – (ta(k) – LCT) (This is the value of previous counter X minus the amount would have leaked.

• If X’ is negative (bucket would be empty), the cell is compliant, X’ is set to 0, X is set equal to X’ + I, and LCT = ta(k).

• If X’ is nonnegative, check if bucket is full. If not full, cell is compliant and do as above.

• If bucket is full, the cell is not compliant, and nothing is updated.

Leaky (token) bucket as a gate

• ir = information rate• bs = burst size

?yes

bs

ir

Let token escape as cell goes by

Tokens enter at given rateCell goes through if bucket has enough tokens

no

input output

Double Leaky BucketInput

Overflow

output

Overflow

Double Leaky Bucket(another arrangement)Input

Overflow

output

Overflow

Double leaky buckets as a pair of gates

• pir = peak information rate• pbs = peak burst size• cir = committed information rate• cbr = Committed burst size

yes yes

cbspbs

pircir

no no

LBP schemes

• LBP=Leaky Bucket Policy

LBP Scheme type 1

Token pool

serverarriving cells

droppedcells

departing cells

• Arriving cells are dropped when no token is present in the token pool

LBP Scheme type 2

Token pool

serverarriving cells departing cells

• Arriving cells are tagged if not complient.

tagged cells

LBP Scheme type 3

Token pool

serverarriving cells

droppedcells

departing cells

• Arriving cells are placed in the queue if no token is present in the token pool

buffer

LBP Scheme type 4

Token pool

serverarriving cells departing cells

• Arriving cells are placed in the queue if no token is present in the token pool. Nonconforming cells are tagged.

buffer

tagged cells

GCRA and ATM Services

• GCRA is used to define usage parameters• CBR (Class A) (one GCRA)

– GCRA(1/PCR, CDVT)• rt-VBR (Class B) (two GCRAs)

– GCRA(1/PCR, CDVT)– GCRA(1/SCR, CDVT+BT)– ATM Forum defines 3 variations based on how the

CLP bit is used.• ABR

– uses GCRA with changing values for parameters.

ATM Conformance Definitions

• VBR.1 – Leaky bucket 1 sends CLP(0+1) compliant cells to leaky bucket 2 and discards the noncompliant cells. Leaky bucket 2 sends CLP(0+1) compliant cells to the network and discards noncompliant cells.

• VBR.2 – Leaky bucket 1 sends CLP(1) compliant cells to the network, sends CLP(0) compliant cells to leaky bucket 2, and discards CLP(0+1) noncompliant cells. Leaky bucket 2 sends CLP(0) compliant cells to the network and discards noncompliant cells.

• VBR.3 – Leaky bucket 1 sends CLP(1) compliant cells to the network, sends CLP(0) compliant cells to leaky bucket 2, and discards CLP(0+1) noncompliant cells. Leaky bucket 2 sends CLP(0) compliant cells to the network and tags noncompliant cells and sends them to the network.